KR20240000646A - Hot rolled steel sheet with high hole expansion ratio and manufacturing process thereof - Google Patents

Abstract

By weight%: 0.15% ≤ C ≤ 0.20%, 0.50% ≤ Mn ≤ 2.00%, 0.25% ≤ Si ≤ 1.25%, 0.10% ≤ Al ≤ 1.00%, where 1.00% ≤ (Al+Si) ≤ 2.00%, 0.001 % ≤ Cr ≤ 0.250%, P ≤ 0.02%, S ≤ 0.005%, N ≤ 0.008%, and optionally one or more of the following elements; A hot rolled steel having a chemical composition comprising 0.005% ≤ Mo ≤ 0.250%, 0.005% ≤ V ≤ 0.250%, 0.0001% ≤ Ca ≤ 0.003% and 0.001% ≤ Ti ≤ 0.025%, with the balance being Fe and inevitable impurities. relates to a sheet, the microstructure comprising, in surface fraction, ferrite and bainite in a sum greater than 5% and strictly less than 20%, with the balance consisting of tempered martensite.

Description

Hot rolled steel sheet with high hole expansion ratio and manufacturing method thereof {HOT ROLLED STEEL SHEET WITH HIGH HOLE EXPANSION RATIO AND MANUFACTURING PROCESS THEREOF}

The invention relates to hot rolled steel sheets having a yield strength of 780 MPa to 1000 MPa, a tensile strength of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa, and a hole expansion ratio of more than 45%, which are used for automotive applications. It can be used in the manufacture of structural parts.

Reducing the weight of vehicles to reduce _CO2 emissions is a major challenge in the automobile industry. These weight savings must meet safety requirements. To meet these requirements, new high-strength steels are continuously being developed by the steelmaking industry. As the use of high strength steels increases in automotive applications, there is a growing demand for steels with both increased strength and improvements in hole expansion performance. Accordingly, several families of steels offering various strength levels have been proposed.

Publication EP 1138796 discloses hot rolled steel sheets with a tensile strength higher than 1000 MPa, usable for automotive parts. The manufacture of these hot rolled steel sheets consists of molybdenum, which, due to its hardening influence, can achieve a fully bainitic structure and high mechanical properties, and vanadium, which can obtain fine nitrides and carbides and a high level of tensile mechanical properties. It requires essential and expensive alloy elements such as

In publication WO 2018108653, hot rolled flat steel sheets with a tensile strength of 800 to 1500 MPa, a yield strength of 700 MPa or more, an elongation of 7 to 25%, and a hole expansion value of more than 20% are produced. Such martensitic hot-rolled steel sheets are produced by the so-called quenching and splitting process, in which the steel sheets are first cooled to the extent that martensitic transformation is incomplete. The steel sheet is then reheated at a temperature range where the carbon splits, i.e. diffuses from martensite, enriching and stabilizing austenite. Afterwards, the steel sheet is cooled to room temperature. Accordingly, the final steel sheet contains split martensite, new martensite and retained austenite. However, specific devices and production lines are required to implement this process.

Publication WO 2012130434 discloses a variable heat treatment across the width of a coated sheet with a dual-phase or martensitic microstructure in order to obtain metal sheets with tailored mechanical properties across the width of the metal strip. However, this method requires specific and dedicated production equipment. Additionally, localized heat treatment may cause residual stress and flatness problems.

One object of the present invention is to provide high strength hot rolled steel sheets without the need to add large amounts of expensive elements.

Another object of the present invention is to manufacture hot rolled steel sheets using conventional production lines without increasing manufacturing costs.

Accordingly, the present invention provides a yield strength of 780 MPa to 1000 MPa, a tensile strength (TS) of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa, a total elongation greater than 8%, and an expansion ratio (HER) greater than 45%. The purpose is to provide a flat hot-rolled high-strength steel having ).

Another object of the present invention is to provide a steel sheet which has a high resistance to crack initiation and propagation, making it possible to avoid any brittle fracture of parts manufactured from the steel sheet. To this end, the present invention aims to provide flat hot-rolled steel sheets with a Charpy V fracture energy at 20° C. of greater than 50 J/cm 2 .

The present invention provides, in weight percent: 0.15% ≤ C ≤ 0.20%, 0.50% ≤ Mn ≤ 2.00%, 0.25% ≤ Si ≤ 1.25%, 0.10% ≤ Al ≤ 1.00%, where 1.00 ≤ (Al + Si) ≤ 2.00, 0.001% ≤ Cr ≤ 0.250%, P ≤ 0.02%, S ≤ 0.005, N ≤ 0.008% and optionally one or more of the following elements; A hot rolled steel having a chemical composition comprising 0.005% ≤ Mo ≤ 0.250%, 0.005% ≤ V ≤ 0.250%, 0.0001% ≤ Ca ≤ 0.0030% and 0.001% ≤ Ti ≤ 0.025%, with the balance being iron and inevitable impurities. relates to a sheet, the microstructure comprising, in surface fraction, ferrite and bainite in a sum greater than 5% and strictly less than 20%, with the balance consisting of tempered martensite.

In a preferred embodiment, the silicon content is between 0.40% and 0.90%.

In another preferred embodiment, the aluminum content is between 0.30% and 0.90%.

In another preferred embodiment, the sum of the aluminum and silicon contents is between 1.20% and 2.00%.

The hot rolled steel sheets of the invention have a yield strength (YS) of 780 MPa to 1000 MPa and a tensile strength (TS) of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa.

According to the invention, the total elongation of the steel is greater than 8%.

According to the invention, the hole expansion value of the steel is greater than 45%.

According to the invention, the Charpy V energy of the steel is higher than 50 J/cm2 at 20°C.

The thickness of the steel of the present invention is 1.8 to 4.5 mm, preferably 1.8 to 3.5 mm.

According to the invention, the hot rolled steel sheet comprises at the surface a ferrite layer having a thickness of less than 5% of the thickness of the hot rolled steel sheet.

According to the invention, the hot rolled steel sheet is coated with zinc or a zinc-based alloy.

In a first embodiment, the zinc-based coating comprises from 0.01 to 8.0% by weight of Al, optionally from 0.2 to 8.0% by weight of Mg, with the balance being Zn.

In a second embodiment, the zinc-based coating comprises 0.15 to 0.40% by weight of Al, the balance Zn.

The present invention provides a method for producing hot rolled steel sheets comprising the following sequential steps:

- providing a steel semi-finished product having the above-mentioned composition,

- hot rolling the steel semi-finished product with a final rolling temperature of 875 ° C to 950 ° C to obtain a steel sheet,

- cooling the steel sheet at a cooling rate (V _R1 ) higher than 50° C./s to obtain a cooled steel sheet,

- coiling at a temperature (T _coil ) below 160° C. and below Mf to obtain a coiled steel sheet,

- heat treating the coiled steel sheet to a heat treatment temperature (θ _A ) for a period (t _A ), wherein θ _A and t _A are such that P _A = θ _A (22 + log ₁₀ t _A ) is 15400 to 17500 The heat treatment step is such that θ _A is expressed as K and t _A is expressed as time.

In a first embodiment of the invention, the heat treatment step of the manufacturing process is carried out by batch processing in an inert or HNX atmosphere at a heat treatment temperature (θ _A ) of 400° C. to 475° C., with a period (t _A ) at said heat treatment temperature. is 10 to 25 h.

In a second embodiment of the present invention, the heat treatment step is carried out in a continuous annealing line up to a heat treatment temperature (θ _A ) of 500 ° C. to 600 ° C., and the period (t _A ) at the heat treatment temperature is 40 s to 100 s, preferably Typically, it is 50 s to 100 s.

In a preferred embodiment of the invention, the P _A parameter is in the range of 15500 to 17000.

The manufacturing process further includes a pickling step after the coiling step and before heat treatment.

The manufacturing process further includes a pickling step after the heat treatment step.

In a first cooling technique embodiment of the invention, cooling is carried out by water cooling at a cooling rate (V _R1 ) higher than 75° C./s.

In the second cooling technique, cooling at a cooling rate (V _R1 ) is carried out until an intermediate temperature (T _i ) of 500 to 550° C. is reached, and then starting from T _i ,

- additional air cooling is performed for a period (t ₂ ) of 1 to 5 seconds,

- The sheet is cooled at a cooling rate (V _R2 ) higher than 40 °C/s.

In a preferred embodiment of the invention, the air cooling is carried out for a period t ₂ of 2 to 3 seconds.

The steel sheet according to the invention can be used for the manufacture of structural parts for automobiles.

The invention will be described in more detail but without introducing limitations with reference to the accompanying drawings.

Figure 1 shows the development of the hole expansion ratio (HER) as a function of the heat treatment parameter P _A = θ _A (22 + log ₁₀ t _A ) for steel compositions according to the invention.
Figure 2 shows the development of tensile strength as a function of the parameter P _A , for steel compositions according to the invention.
Figure 3 shows an example of the microstructure of a hot rolled steel sheet according to the invention.
Figure 4 shows an example of the microstructure of a hot rolled steel sheet that does not fall under the present invention.
Figure 5 shows the microstructure of one embodiment according to the invention, wherein the steel sheet contains a ferrite layer on its surface.

In the following description of the present invention, the yield stress (YS), tensile strength (TS) and total elongation of the steel sheet represent the standard JIS Z2241. Hole expansion ratio (HER) refers to the standard ISO 16630:2009.

To reach the desired microstructural and mechanical properties, chemical composition and processing parameters are important. The steel composition in weight percent is as follows:

- 0.15% ≤ C ≤ 0.20%: If the carbon content is less than 0.15%, the tensile strength of 950 MPa may not be reached. When the carbon content exceeds 0.20%, the yield strength and tensile strength may exceed 1000 MPa and 1150 MPa, respectively, and the total elongation may be lower than 8%.

- 0.50% ≤ Mn ≤ 2.00%: If the manganese content is less than 0.50%, the quenchability of the steel decreases, and the sum of the ferrite and bainite surface fractions cannot strictly be lower than 20%, so the tensile strength is 950. It can be lower than MPa. If the manganese content exceeds 2.00%, the risk of center segregation increases, which is detrimental to the yield strength, tensile strength and hole expansion values.

- 0.25% ≤ Si ≤ 1.25%: Silicon is an element used to achieve deoxidation and solution hardening in the liquid phase. If the Si content is less than 0.25%, the hardenability of the steel decreases. However, when Si exceeds 1.25%, the kinetics of carbide formation decreases. Accordingly, the ferrite content may be higher than 20% and the tensile strength may be lower than 950 MPa. In a preferred embodiment, the silicon content is between 0.40% and 0.90%.

- 0.10% ≤ Al ≤ 1.00%: Addition of aluminum contributes to efficient deoxidation in the liquid phase and is advantageous for stabilizing ferrite. If the content of aluminum is less than 0.10%, the sum of the ferrite and bainite surface fractions of the hot rolled sheet may be lower than 5%, so that the total elongation of the sheet may be lower than 8%. If it exceeds 1.00%, too much ferrite may be formed upon cooling, making it impossible to achieve the yield and tensile strength levels required by the present invention. In a preferred embodiment, the aluminum content is between 0.30% and 0.90%.

- 1.00 ≤ Al + Si ≤ 2.00: When the sum of the contents of silicon and aluminum is 1.00% to 2.00%, a microstructure containing ferrite and bainite in more than 5% and less than 20% can be obtained, and thus increased Ductility and elongation can be obtained. In a preferred embodiment, the sum of the silicon and aluminum contents is between 1.20% and 2.00%, in order to promote the formation of a ferrite layer on the main surface of the steel sheet. The ferrite layer can achieve a bending radius divided by the sheet thickness that is lower than 1 in the rolling direction and lower than 1.5 in the transverse direction.

- P ≤ 0.02%: If the phosphorus content exceeds 0.02%, segregation may occur at grain boundaries and the elongation of the steel sheet may decrease. Additionally, phosphorus at these high amounts can cause temper embrittlement when the coiled steel sheet is subjected to further heat treatment. Preferably, the phosphorus content is higher than 0.0005% because it is expensive to achieve low levels of phosphorus content in steel mills, without corresponding significant benefits for mechanical properties.

- S ≤ 0.005%: Sulfur content is limited to 0.005% to reduce sulphide formation, which is detrimental to sheet ductility. Preferably, the sulfur content is higher than 0.0005%, since it is very expensive to achieve low levels in steelmaking, without corresponding significant benefits for mechanical properties.

- N ≤ 0.008%: When the nitrogen content exceeds 0.008%, certain elements may precipitate in the form of nitrides or carbonitrides in liquid or solid state. Coarse deposits must be prevented because they reduce the ductility of hot rolled steel sheets. Preferably, the nitrogen content is higher than 0.001%. However, lowering the nitrogen content to less than 0.001% is expensive and does not significantly improve the mechanical properties.

- 0.001% ≤ Cr ≤ 0.250%: Chromium improves hardenability. If the Cr content is less than 0.001%, hardenability cannot be obtained. If Cr exceeds 0.250%, the risk of macro and micro segregation increases, so the tensile strength may be lower than 950 MPa.

- 0.005% ≤ Mo ≤ 0.250%: Molybdenum can be added as an optional element to increase hardenability, that is, to more easily obtain martensite formation upon cooling. Below 0.005%, this effective effect is not achieved. However, molybdenum is an expensive element, so its content is limited to 0.250%, so that the production of steel sheets is cost-effective.

- 0.005% ≤ V ≤ 0.250%: Vanadium is an optional element, and a steel sheet with high toughness can be obtained after batch heat treatment. However, addition of more than 0.250% is not cost-effective.

- 0.0001% ≤ Ca ≤ 0.0030%: Calcium may be added as an optional element. Adding Ca in the liquid phase can produce fine oxides or oxysulfides. These particles act as nucleents for subsequent fine precipitation of titanium nitride/carbonitride. By reducing the size of carbonitride, improved pore expansion ability can be achieved.

- 0.001% ≤ Ti ≤ 0.025%: Titanium can also be added as an optional element: when titanium is higher than 0.025%, it is liable to precipitate in the liquid phase in the form of coarse titanium nitride, which reduces sheet ductility. However, reducing titanium to levels lower than 0.001% is difficult on an industrial scale and has no additional effect on mechanical properties.

The remainder of the composition is iron and inevitable impurities resulting from smelting.

Now, the microstructure of the hot rolled steel sheet according to the present invention will be described in detail.

According to the invention, the sum of ferrite and bainite is greater than 5% and strictly less than 20%. Unless this sum is strictly lower than 20%, the yield strength and tensile strength decrease and cannot reach the minimum values of 780 MPa and 950 MPa, respectively. Moreover, the hole expansion ratio will be low. At less than 5% ferrite and bainite, the ductility of the steel sheet decreases.

caniques et thermochimiques des aciers”, PYC Edition, 1992, 190 쪽 - 191 쪽에 기재되어 있는 소위 마르텐사이트 템퍼링의 단계 3 에 대응한다.The remainder of the microstructure consists of tempered martensite. Within the framework of the present invention, tempered martensite is defined as recovered martensite containing precipitated cementite that can be fused at the highest tempering temperature. Its features are in the publication of A. Constant, G. Henry, and JC Charbonnier: “Principes de bases des traitements thermiques, thermom

caniques et thermochimiques des aciers”, PYC Edition, 1992, pages 190 - 191, corresponding to stage 3 of the so-called martensitic tempering.

According to the invention, steel sheets are manufactured through a hot rolling process. This makes it possible to obtain a steel sheet with two main parallel surfaces and an opposing surface, which also has edges that can be designated as secondary surfaces. According to an embodiment of the invention, a hot rolled steel sheet comprises a ferrite layer at the major surface, having a thickness of less than 5% of the thickness of the hot rolled steel sheet.

The process for manufacturing hot rolled sheets is now described.

The semi-finished product to be further hot rolled is provided with the steel composition described above. These semi-finished products may be in the form of ingots or slabs obtained by continuous casting, and are typically about 200 mm thick. Alternatively, these semi-finished products may also be in the form of sheets or thin slabs with a thickness of the order of tens of millimeters, obtained by direct casting between counter-rotating rollers. The semi-finished product can be heated to a temperature above 1150°C to facilitate hot rolling, with a final hot rolling temperature of 875°C to 950°C. Hot rolling at temperatures below 875°C promotes austenite and then induces excessive formation of ferrite upon cooling, reducing formability. When the hot rolling temperature exceeds 950°C, the tendency to generate scale increases, resulting in poor surface quality of the product.

The hot rolled product is then cooled at a cooling rate (V _R1 ) of at least 50 °C/s to prevent ferrite formation until the coiling temperature is below 160 °C and also below Mf, where Mf is Indicates the temperature at which the transformation of nite to martensite ends. According to Malcom Blair and Thomas L Stevens, "Steel castings Handbook - 6th edition", the martensite termination temperature (Mf) is 245°C lower than the martensite start temperature (Ms), Journal of the Iron and Steel Institute, It can be calculated from the formula derived by Andrews published in 203, 721-727, 1965:

Ms(℃) = 785 - 453%C - 16.9%Ni - 15%Cr - 9.5%Mo + 217 (%C)² - 71.5%C ×%Mn - 67.6%C ×%Cr

In a preferred embodiment, the hot rolled product is coiled at a temperature below 160° C. and below (Mf-10° C.). In this way, high microstructural homogeneity is achieved along all steel strips.

In one embodiment of the cooling technique, the cooling step includes cooling higher than 75° C./s to obtain a martensitic microstructure matrix containing ferrite and bainite with a combined surface area greater than 5% and strictly less than 20%. This is accomplished by single-stage cooling with water cooling at speed (V _R1 ).

In another embodiment of the cooling technique, the cooling step is carried out by multi-stage cooling, with a first cooling step at the cooling rate (V _R1 ) to reach an intermediate temperature (T _i ) of 500 to 550° C. Thereafter, air cooling is carried out immediately for a period (t ₂ ) of 1 to 5 seconds, preferably 2 to 3 seconds, before the last cooling step at a cooling rate higher than 40° C./s. Multistage cooling makes it possible to achieve partial ferrite or bainite transformation, thus obtaining 5 to 20% ferrite plus bainite within the martensitic matrix.

Whatever the cooling technique, the hot rolled steel is then heat treated after cooling at temperature (θ _A ) for a period (t _A ), where t _A represents the period at temperature (θ _A ), θ _A and t _A is the heat treatment parameter, P _A = θ _A (22 + log ₁₀ t _A ) is set to 15400 to 17500. Therefore, P _A takes into account the combined thermal effects of temperature and period.

From the prior art, some mechanical properties such as hole expansion value and total elongation are improved with high values of parameter P _A. Conversely, as the parameter P _A increases, the yield strength and tensile strength values decrease. For martensitic steel, publication WO 2012130434 discloses that mechanical properties are optimal when P _A is 13000 to 15000. In particular, the hole expansion value continuously increases with P _A. In a surprising way, as shown in Figure 1, the present invention provides evidence that the pore expansion value decreases by a significant percentage below the P _A specific value of ~16000. Therefore, as shown by Figures 1 and 2, the present invention makes it possible to obtain the desired mechanical properties when the P _A value is in the range of 15400 to 17500, especially 15500 to 17000.

According to the invention, the heat treatment step can be carried out in a discontinuous (batch) or continuous manner.

In a first embodiment of the invention, the heat treatment step of the manufacturing process is carried out by batch processing the coils of hot rolled sheet in a furnace with an inert or HNX atmosphere at a heat treatment temperature (θ _A ) of 400° C. to 475° C. The period at this heat treatment temperature (t _A ) is 10 to 25 h to obtain a tempered martensitic matrix combining good formability and tensile properties.

In a second embodiment of the present invention, the heat treatment step is carried out on a continuous annealing line up to a heat treatment temperature (θ _A ) of 500° C. to 600° C., and the period (t _A ) at the heat treatment temperature provides good formability and tensile properties. 40 s to 100 s, preferably 50 s to 100 s, to obtain a tempered martensite matrix that combines.

A first pickling step may be added after coiling and a second pickling step may be added after heat treatment to remove surface oxides.

It is known from the prior art that martensitic steels that are subsequently tempered and cooled slowly can exhibit low toughness. In the present invention, the steel composition and heat treatment conditions are specified to obtain a Charpy V energy of at least 50 J/cm 2 at 20° C. on the final hot rolled steel sheet. Therefore, the obtained steel sheet has no temper embrittlement.

The thickness of the hot rolled steel sheet is typically 1.8 to 4.5 mm, preferably 1.8 to 3.5 mm.

The invention is now illustrated by the following examples in a non-limiting way.

Example 1

Semi-finished products in cast form with a thickness of 28 to 40 mm were provided with the composition detailed in Table 1. For different compositions, the calcium content was 0.002 wt%, the remainder of the composition being iron and impurities due to smelting. The martensite end temperature was calculated from the value of the martensite start temperature as follows: Mf = Ms - 245 °C. These semi-finished products were heated at a temperature above 1150° C. and further hot rolled to a thickness of 1.8 to 4.5 mm. Table 2 details the manufacturing conditions applied. Tests 1-15 correspond to the first cooling technique embodiment described above, and Tests 16-18 correspond to the second cooling technique conditions described above. The pickling step was performed after coiling and after heat treatment. In tests 4 and 9, hot rolled steel sheets are galvanized (GI).

The microstructure of heat-treated steel sheets was determined on polished specimens etched with Nital and observed by optical and scanning electron microscopy. The surface fractions of different components of the microstructure were determined through image analysis combined with quantification. Additionally, the final presence of a ferrite layer on the main surface of the steel sheet was evaluated. The ratio of components and the thickness of the final ferrite layer are shown in Table 3. Table 4 summarizes the mechanical properties of the final heat-treated steel sheet. Yield stress (YS), ultimate tensile strength (TS) and total elongation were determined according to standard JIS Z2241. The hole expansion ratio was determined according to ISO 16630:2009.

Charpy V energy was measured for sub-thickness size specimens at 20°C, and the measured fracture energy was divided by the ligament area under the V notch of the test specimen.

The hole expansion method measures the initial diameter (Di) (nominal: 10 mm) of the hole before stamping and then the final diameter of the hole after stamping, which is determined when through cracks are observed in the thickness direction of the sheet on the edges of the hole. This consists in measuring the diameter (Df). The hole expansion capacity Ac% is determined according to the following equation: Ac = 100*(Df-Di)/Di. Accordingly, Ac is used to quantify the ability of a sheet to withstand stamping at the level of the cut orifice.

In Tests 1-7 and 16-17, the composition and production conditions correspond to the present invention. Accordingly, the desired microstructure is obtained. Figure 3 shows the microstructure obtained in Test 7 containing 89% tempered martensite and 11% ferrite and bainite. As a result, high tensile properties and high hole expansion ratios are obtained. The sheet has high toughness because the Charpy energy at 20°C far exceeds 50 J/cm2.

In Test 1-3, Test 6-7, and Test 16-17, a ferrite layer exists on the main surface of the steel sheet, and higher bending properties can be achieved. In particular, for Test 7, the bending radius divided by the sheet thickness was lower than 1 in the rolling direction and lower than 1.5 in the transverse direction, indicating excellent bending properties.

Figures 5 (a) and (b) show the ferrite layer present on two opposing major surfaces of the steel sheet of Test 7, respectively, among the sheets produced.

Tests 8-11 and Test 18 do not correspond to the manufacturing conditions of the present invention. As a result, heat-treated steel sheets do not meet the required mechanical properties.

In fact, in Test 8, the coiling temperature is higher than 160° C. and exceeds the martensite termination transformation temperature. Therefore, an excessive amount of ferrite is generated, reducing the tensile strength value and hole expansion ratio.

In tests 9 and 10, the parameter P _A exceeds 17500 and the batch heat treatment temperature exceeds 475 °C. 80% of tempered martensite is present in the final microstructure, so the tensile strength does not correspond to 950 MPa.

In Test 11, the finishing hot rolling temperature is less than 875°C. Therefore, austenite is promoted and excessive ferrite is generated upon cooling. Figure 4 shows the microstructure obtained in Test 11 containing 60% tempered martensite and 40% ferrite and bainite. Therefore, the yield strength, tensile strength and hole expansion are insufficient.

In Test 18, the median duration (t ₂ ) of the cooling technique is higher than 5 s. Therefore, excessive amounts of ferrite and bainite are produced, reducing yield strength, tensile strength and hole expansion values.

In tests 12-15, the steel composition is outside the scope of the present invention. Therefore, the final steel sheet does not match the mechanical and microstructural characteristics.

In Test 12, the carbon, manganese and silicon contents of the steel composition exceed the values specified by the invention. Therefore, insufficient amounts of ferrite and bainite are present and the hole expansion properties are insufficient.

On the other hand, in test 13, insufficient values of tensile strength and hole expansion are obtained because the carbon content is lower than 0.15%.

In test 14, the carbon, silicon, aluminum and chromium contents of the steel are not according to the invention. In particular, because the carbon content is low, excessive amounts of ferrite and bainite are generated, making it impossible to obtain sufficient tensile stress hole expansion values.

Finally, in test 15, the manganese content is higher than 2%. Therefore, insufficient amounts of ferrite and bainite are obtained, and the hole expansion value does not reach 45%.

Therefore, the steel sheet according to the invention can be used with benefit for manufacturing structural parts of automobiles.

Claims (23)

By weight%,
0.15% ≤ C ≤0.20%
0.50% ≤ Mn ≤2.00%
0.25% ≤ Si ≤ 1.25%
0.10% ≤ Al ≤ 1.00%,
1.00% ≤ (Al+Si) ≤ 2.00%,
0.001% ≤ Cr ≤ 0.250%
P ≤ 0.02%
S ≤ 0.005%
N ≤ 0.008%
and optionally one or more of the following elements:
0.005% ≤ Mo ≤ 0.250%
0.005% ≤ V ≤ 0.250%
0.0001% ≤ Ca ≤ 0.003% and
0.001% ≤ Ti ≤ 0.025%,
A hot rolled steel sheet having a chemical composition comprising Fe and the balance being unavoidable impurities,
A hot rolled steel sheet, the microstructure comprising, in surface fraction, ferrite and bainite in a sum greater than 5% and strictly less than 20%, with the balance consisting of tempered martensite. According to claim 1,
Hot rolled steel sheet with a Si content of 0.40% to 0.90%. The method of claim 1 or 2,
Hot rolled steel sheet, wherein Al is 0.30% to 0.90%. The method according to any one of claims 1 to 3,
Hot rolled steel sheet with Al+Si content of 1.20% to 2.00%. The method according to any one of claims 1 to 4,
A hot rolled steel sheet having a yield strength (YS) of 780 MPa to 1000 MPa and a tensile strength (TS) of 950 MPa to 1150 MPa. The method according to any one of claims 1 to 5,
Hot rolled steel sheet with a total elongation greater than 8%. The method according to any one of claims 1 to 6,
Hot rolled steel sheet with a hole expansion ratio (HER) higher than 45%. The method according to any one of claims 1 to 7,
Hot rolled steel sheet with Charpy V energy higher than 50 J/cm² at 20°C. The method according to any one of claims 1 to 8,
Hot rolled steel sheet with a thickness of 1.8 to 4.5 mm. The method according to any one of claims 1 to 9,
A hot rolled steel sheet comprising a ferrite layer at the surface having a thickness of less than 5% of the thickness of the hot rolled steel sheet. The method according to any one of claims 1 to 10,
A hot rolled steel sheet, wherein the hot rolled steel sheet is coated with zinc or a zinc-based alloy. According to claim 11,
The zinc-based coating comprises 0.01 to 8.0% by weight of Al, optionally 0.2 to 8.0% by weight of Mg, the balance Zn. According to claim 11,
A hot-rolled steel sheet, wherein the zinc-based coating comprises 0.15 to 0.40% by weight of Al, the balance Zn. A method for producing hot rolled steel sheets, comprising the following sequential steps:
- providing a steel semi-finished product having a composition according to any one of claims 1 to 4,
- hot rolling the steel semi-finished product with a final rolling temperature of 875 ° C to 950 ° C to obtain a steel sheet,
- cooling the steel sheet at a cooling rate (V _R1 ) of at least 50° C./s to obtain a cooled steel sheet,
- coiling at a temperature (T _coil ) below 160° C. and below Mf to obtain a coiled steel sheet,
- heat treating the coiled steel sheet at a heat treatment temperature (θ _A ) for a period (t _A ), wherein θ _A and t _A are such that P _A = θ _A (22 + log ₁₀ t _A ) is 15400 to 17500 The heat treatment step is such that θ _A is expressed in K units, and t _A is expressed in time units.
A method of producing a hot rolled steel sheet comprising. According to claim 14,
The step of heat treating is carried out by batching in an inert or HNX atmosphere at a heat treatment temperature (θ _A ) of 400° C. to 475° C., and the period (t _A ) at the annealing temperature is 10 to 25 h. How to manufacture sheets. According to claim 14,
The heat treatment step is performed at a heat treatment temperature (θ _A ) of 500 ° C. to 600 ° C. on a continuous annealing line, and the period (t _A ) at the heat treatment temperature is 40 s to 100 s, producing a hot rolled steel sheet. How to. The method according to any one of claims 14 to 16,
P _A is 15500 to 17000. A method for producing hot rolled steel sheets. The method according to any one of claims 14 to 17,
A method of producing a hot rolled steel sheet, further comprising a pickling step after the coiling step and before the heat treating step. The method according to any one of claims 14 to 18,
A method of producing a hot rolled steel sheet, further comprising a pickling step after the heat treating step. The method according to any one of claims 14 to 19,
The method of producing a hot rolled steel sheet, wherein the cooling is carried out by water cooling, and V _R1 is higher than 75° C./s. The method according to any one of claims 14 to 20,
The cooling step at the above cooling rate (V _R1 ) is carried out to reach an intermediate temperature (T _i ) of 500 to 550° C., and then
- an additional air cooling step is performed for a period of 1 to 5 seconds (t ₂ ), after which
- A method for producing hot rolled steel sheets, wherein the steel sheets are cooled at a cooling rate (V _R2 ) higher than 40° C./s. According to claim 21,
The method of producing a hot rolled steel sheet, wherein the air cooling step is performed for a period (t ₂ ) of 2 to 3 seconds. Use of hot rolled steel sheets according to any one of claims 1 to 13 or according to any one of claims 14 to 22 for the manufacture of structural parts of vehicles.